Many different types of photovoltaic devices are known in the art (e.g., see U.S. Patent Document Nos. 2004/0261841, 2006/0180200, U.S. Pat. Nos. 4,335,266, 4,611,091, 6,784,361, 6,288,325, 6,631,603, and 6,123,824, the disclosures of which are incorporated by reference herein in their entireties). Examples of known photovoltaic devices include CIGS (approximately Cu(In, Ga)(Se,S)2 and/or CuInx-1GaxSe2) solar cells. CIGS films are conductive semiconductor compounds that are often referred to as an absorber or light absorbing layer(s). Generally speaking, CIGS type photovoltaic devices include, from the front (or light incident) side moving rearwardly, a front transparent cover sheet (or substrate) such as glass, a front electrode comprising a transparent conductive layer(s) (e.g., a transparent conductive oxide), a light absorption semiconductor film (e.g., CIGS), a rear electrode or contact, and a rear substrate of a material such as, for example, glass (or metal foil for certain example flexible applications). In some instances, an adhesive may be provided between the front substrate and the front electrode. It is also the case in some instances that the device is provided with window layer(s) (e.g., of or including CdS, ZnO, or the like). Photovoltaic power is generated when light incident on the front side (or front substrate) of the device passes through the front electrode and is absorbed by the light absorption semiconductor film (e.g., CIGS), as is known in the art.
For example, with reference to FIG. 1, there is generally provided a schematic cross-sectional diagram illustrating various elements of a CIGS-type photovoltaic device 10. The cell 10 is structurally supported on a rear glass substrate (or back glass) 12. A back contact made up of a metal layer, such as, for example, molybdenum (Mo) 14 is typically deposited on the rear glass substrate 12. The first active region of the device 10 comprises a semiconductor layer 16 which is typically a p-type copper indium/gallium diselenide (CIGS). A thin “window” layer of n-type compound semiconductor 18, typically comprising cadmium sulfide (CdS) may then be deposited on CIGS layer 16. A front electrode 20 is deposited on the CdS layer 18 and acts as a transparent front contact for the photovoltaic device 10. The device 10 may be completed by including a series of front face contacts (not shown) in the form of, for example, a metal grid on top of the transparent front contact 20 to facilitate the extraction of generated electrons, and a front glass substrate 22. A large solar cell may also be divided into a number or smaller cells by means of scribes, such as, for example, laser or mechanical scribes or the like, traditionally referred to as P1, P2 and P3, which allow individual cells to be connected in series.
As noted above, a metal such as Mo may be used as the rear electrode (or back contact) 14 of a photovoltaic device, such as, for example, a CIGS solar cell 10, to extract positive charges generated in the CIGS semiconductor absorber 16 of the solar cell 10. In certain instances, the Mo rear electrode 14 may be sputter-deposited using, for example, direct-current magnetron sputtering, onto the back glass substrate 12 of the CIGS solar cell 8. However, using Mo alone as the material for the back contact 14 of the solar cell 10 suffers from certain disadvantages. For example, Mo has a relatively high material cost and sometimes suffers from the problem of delamination from the back glass substrate. In addition, Mo typically exhibits relatively low conductivity and suffers from a relatively slow deposition rate, and thus causes correspondingly low production line throughput. As a result, using Mo as the material for the back contact accounts for a substantial portion of the total device cost. Moreover, one of the preferred requirements for a back contact for CIGS type solar cells is to achieve a sheet resistance (SR) on the order of about <1 ohm/square. In order for a Mo back contact to meet this requirement, the Mo thickness must typically range from 300-900 nm, depending upon the CIGS deposition method. In other words, the thicker the film, the lower its sheet resistance. This results in 15-25% of the total photovoltaic module cost.
Attempts have been made to use other types of metals as the sole material to form the back contact. These attempts have resulted in limited success with only a few other materials, such as, for example, tungsten (W), tantalum (Ta) and niobium (Nb), exhibiting sufficient compatibility with the CIGS absorber, e.g., not reacting with the CIGS absorber. However, these alternatives result in a lower CIGS device efficiency as compared to devices using Mo as the material for the back contact.
Unsuccessful attempts have been made to substitute Cu as the material for a back contact in a CIGS device. The Cu/CIGS stack was found to peel off during CIGS deposition. These attempts used methods, such as, for example, molecular beam epitaxy (MBE) for CIGS deposition. MBE is very different from the much faster processes currently being used for CIGS production, such as, for example, co-evaporation, sputtering, electroplating, and the like. In addition, these attempts used CIGS deposition at high temperatures instead of methods that use unheated deposition followed by high-temperature selenization. Moreover, the prior attempts failed to recognize advantages that could be realized by taking measures to mitigate stress between Cu and Mo, and to provide improved adhesion of Cu to the back glass substrate, as is provided by certain example embodiments disclosed herein.
In certain example embodiments of this invention, there is provided a photovoltaic device (e.g., solar cell) comprising: a front cover glass (substrate); a semiconductor absorber film; a back contact comprising a first conductive layer comprising or consisting essentially of copper and a second conductive layer comprising or consisting essentially of molybdenum; and a rear substrate; wherein the first conductive layer comprising of consisting essentially of copper is located between at least the rear substrate and the second conductive layer comprising or consisting essentially of molybdenum, and wherein the semiconductor absorber film is located between at least the back contact and the front substrate.
In certain example embodiments disclosed herein, a substantial portion of the Mo back contact is substituted with copper (Cu), which is a less expensive and a more conductive alternative. It has been found that substituting Cu for a portion of the Mo of the back contact does not significantly compromise cell performance in a CIGS cell (of course, it will be understood that Cu used in back contact configurations may include certain trace or minor amounts of other elements of materials that do not substantially affect the performance or electrical characteristics of Cu, and as discussed herein, Cu may include an oxidation graded layer of Cu). The many advantages of using Cu as the bulk (or at least part) of the back contact include, for example, lower cost—Cu sputtering targets are typically at least two times less expensive than using Mo sputtering targets. Additionally, the sputter deposition rates of Cu are typically at least two times greater than that of Mo. For example, and without limitation, deposition rates for Mo tend to range from about 2.8 to 3.6 nm/kW, while deposition rates for Cu tend to range from about 7.2 to 8.2 nm/kW, thereby providing improvements in throughput and thus more efficient and faster manufacturing. Another advantage of using Cu in the back contact is that sputtered Cu has 4 to 5 times greater conductivity as compared to sputtered Mo. For example, and without limitation, sputtered Cu conductivity may be about 60.7×106 Sm−1, as compared to 15.5×106 Sm−1 for Mo. Therefore, in order to reach substantially the same sheet resistance as a simple Mo back contact, a multi-layer back contact configuration including Cu and Mo could be made for example 4 to 5 times thinner than using Mo alone. Moreover, Cu is compatible with the CIGS absorber since Cu is a main component of the CIGS absorber. The Cu is also separated from the CIGS absorber by the Mo layer, so only traces of Cu from the back contact may reach the CIGS absorber due to heating/diffusion without substantially altering its stoichiometry. It will also be understood that the Cu may include trace or minor amounts of other elements or materials that do not substantially affect the performance or electrical properties of Cu.
In further example embodiments disclosed herein, a stress matching or adhesion layer of or including copper oxide (e.g., CuOX) may be formed between the back glass substrate and a copper based layer. Optionally, the copper based layer may be an oxidation graded copper layer having greater oxidation at a portion adjacent the back glass substrate, such that the portion having the greater oxygen concentration can provide a stress matching function of a copper oxide stress matching layer. A conductive Mo layer is deposited on top of the conductive copper layer. One or both of the Cu and/or Mo layers may be reflective layers in certain example embodiments. It will be understood in example embodiments including an oxidation graded conductive layer, the grading need not be continuous, and may be discontinuous. By providing a stress matching/adhesion layer and/or oxidation graded Cu layer, improved adhesion of the Cu layer to the back substrate may be achieved.
According to an example embodiment disclosed herein, to mitigate mechanical problems that may be associated with a mismatch of the coefficient of thermal expansion (CTE) between layers, a MoCu alloy may optionally be provided as a CTE matching layer between the Mo and Cu of the multi-layer back contact configuration.
In accordance with these and other example embodiments disclosed herein, the portion of the back contact between the Cu and the CIGS absorber continues to be of or including Mo. The use of Mo between the Cu and the CIGS absorber has numerous operational and structural advantages. Among these advantages is that Mo forms a thin (e.g., 3-50 nm, depending on Mo quality parameters, such as, for example, density, purity, stress, etc.) molybdenum selenide (MoSe2) layer at its interface with the CIGS absorber during the high-temperature selenization process used to form the CIGS absorber. The formation of a MoSe2 layer is beneficial for the CIGS device in that it results in the formation of an ohmic (non-rectifying) contact to the CIGS absorber which, in turn, facilitates hole extraction with minimum losses. Another advantage of continuing to use Mo between the Cu and the CIGS absorber is that Mo works well with the optional second mechanical scribe in the CIGS photovoltaic module manufacturing process. Sputter deposited Mo also produces a specific surface morphology that is beneficial for CIGS growth and particularly for the formation of crystallites with large grains sizes that result in high carrier mobility and thus, higher device efficiency.
These and other example embodiments and example advantages are described herein with respect to certain example embodiments and with reference to the following drawings in which like reference numerals refer to like elements, and wherein: